NEC in the Facility, February 23, 2010

One possible problem in this issue's Electrical Troubleshooting Quiz is an isolated ground (IG)

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One possible problem in this issue's Electrical Troubleshooting Quiz is an isolated ground (IG).

Some equipment manufacturers “require” an IG receptacle.

Someone may install an IG to solve power quality problems.

Often, IG installations are wired incorrectly and produce new problems without solving the old ones.

One IG miswiring scheme involves running the IG equipment grounding conductor (EGC) to a separate ground rod. Because this electrode isn't wired to the EGC of the source (service or derived system), it creates a potentially lethal difference of potential.

If your IG is wired “with separate ground” and you plug a copier into it, you could have several hundred volts between the copier and a nearby metallic object. What happens if someone grabs a metal partition frame with one hand and touches the copier frame with the other?

An IG receptacle is one in which the grounding terminal is insulated from the receptacle mounting means. Although the EGC for this receptacle can pass through panelboards without a connection to the panelboard grounding terminal, you must eventually connect it to the EGC terminal of the source [250.146(D)].

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